The present invention resides in a falling pond method for maintaining a salt density gradient in a non-convecting salt gradient pond, i.e., falling solar pond, and means for carrying out said method.
A non-convecting solar pond is an efficient and relatively inexpensive energy collection and storage system. The design of a solar pond is such that it takes advantage of several important properties of water, namely, high heat capacity, transparency to visible and ultraviolet light, opacity to infrared radiation and poor heat conductivity.
The general principles involved in designing a solar pond are relatively simple. A body of water collects large amounts of heat from the sun. Ordinarily the water temperature remains close to the ambient air temperature because the heating of the water produces a convection circulation which brings the absorbed heat to the surface where it is dissipated into the air, largely by evaporation. It has been found that by establishing a salt density gradient which increases with depth, convection circulation can be inhibited thereby greatly reducing loss of heat at the surface of the pond.
For most ordinary uses, such as space heating or industrial process heating, a solar pond should be between two and three meters deep and at least a few hundred square meters in size. The top layer of the pond has little or no salt dissolved therein and the concentration of salt increases with depth until a density gradient layer is established which is between one and one and a half meters deep. Below this gradient region is the heat storage region which is generally of constant density equal to the density of the lowermost region of the salt gradient region. The storage region may be in direct contact with the gradient region or separated therefrom by means of transparent membrane such as plastic or the like in which case the storage region could be salt free. The choice between a direct contact or a separate storage region depends on the relative cost of the salt and plastic. In either case, convection is permitted and in fact is desirable in the heat storage region.
While salt gradient solar ponds are the most cost effective solar thermal system and can provide useful heat at a cost that is less than most conventional methods, the concept has remained largely undeveloped. The principal reason for this lack of development resides in the problems encountered in attempting to maintain the salt density gradient which, if left alone, tends to diffuse away leaving a pond of uniform salinity thereby resulting in convection circulation and a corresponding loss of heat.
Heretofore, one method used to maintain the salt gradient has been simply to add salt to the bottom layers of the pond while flushing the top layers of brine away with fresh water. This method, while simple, suffers from a number of disadvantages the most important of which are added cost for the additional salt and lack of automatic means for determining when salt must be added. While the movement of salt upward from the bottom layers is rather slow, on the order of 0.3 mm per day, a large amount of salt, about 18,000 kg per year, is still needed in order to maintain the proper density gradient in a quarter acre pond. The added expense for the salt coupled with the need for a continual personal surveillance in order to predict when salt must be added has prohibited this method for becoming commercially feasible.
A second method previously employed to maintain the salt gradient in a solar pond comprises removing the top layer of water from the pond which has become salty due to diffusion and transporting it to a holding evaporation pond where the water is allowed to evaporate after which the more concentrated salt solution is returned to the bottom layer of the pond. While this method overcomes the disadvantage of requiring extra salt to maintain the gradient, it requires a large amount of space for the evaporation pond and still requires constant surveillance on the part of personnel in order to determine when the salt water must be removed and returned.
A third method which has been suggested by Dr. Harry Tabor uses a flash evaporator to perform the same function as the evaporation pond discussed above. This particular method has been untried to date due to the considerable expense of the flash evaporator and the complicated system required to employ same. Again, as with the previously discussed methods constant surveillance is required to determine when salt should be added. In addition to the foregoing drawbacks, all of the above-noted methods correct the salt gradient only after a significant amount of salt has reached the surface, i.e. when the salinity gradient has already decayed substantially thereby increasing the likelihood of convection.
Naturally, it would be highly desirable to provide a method for maintaining a substantially constant salt gradient in a solar pond which eliminates the extra cost of adding additional salt and at the same time automatically controls the salt gradient thereby eliminating any convective heat loss.
Accordingly, it is the principal object of the present invention to provide a method for maintaining a substantially constant salt density gradient in a non-convecting solar pond.
It is a further object of the present invention to provide a method as outlined above which is fully automatic.
It is a still further object of the present invention to provide a method as outlined above which is of simple and inexpensive construction.
Further objects and advantages of the present invention will appear hereinbelow.